Electronic sensors and related control devices are widely used in the field of industrial controls. For example, photoelectric sensors are used for detecting the presence or absence of an object and for providing a signal or switching accordingly.
Binary sensors are those sensors in control devices whose outputs are always in one of two states, ON or OFF. This distinguishes them from sensors and control devices whose outputs are analog voltages or currents.
Binary sensors have traditionally been supplied with two outputs, each of which is complementary to the other. When one of the outputs is ON (conducting) the other output is OFF (non-conducting). One output causes operation of the load when, for example, the sensor sensed a product present, and the other output causes operation of the load when, in this example, the sensor sensed the absence of a product.
The increasing use of programmable logic controllers ("PLC's") and shop floor computers has somewhat obviated the need for complementary outputs, since computers may be easily programmed to create internally the complement of an input signal. Nonetheless, there are still many customers who prefer to buy sensors with complementary outputs, and manufacturers of sensors will need to supply complementary outputs for some time to come.
In addition to the increasing use of PLC's on the shop floor, customers are requesting alarm and warning functions built into the sensors to anticipate problems and initiate maintenance of the sensors. Thus, many new sensors are being supplied with a control output that represents the binary status of a sensor (ON or OFF), and a second alarm output that alerts the user to a sensor problem. In general, the Alarm Signal is also annunciated with an indicator on a sensor itself.
The present invention serves with the same device both the customer who wants complementary outputs and the customer who wants an alarm output. In general, those customers using a PLC will have many sensors in their system and will choose the alarm mode so that they can connect the alarm outputs into extra PLC input boards and thus monitor the alarm condition of each sensor. Those customers not using PLC's are more likely to need the complementary output and are willing to give up the alarm output, since they use very few sensors, and since the alarm condition is still annunciated on the sensor.
A device which serves both customers who want a current sourcing output and those who want a current sinking output is disclosed in U.S. Pat. No. 4,982,107, assigned to the assignee of the present invention and incorporated herein by reference. The user selects the output signal to be a sourcing or sinking current by the polarity of the supply voltage.
One solution to providing both complementary outputs and an alarm output in the same device would be to provide a circuit with a third output, such that the user had the sensor output, its complement, and the alarm output. Likewise, a switch could be used to select whether the secondary output was a complement or an alarm. Both of these solutions, however, require more and larger circuitry than the present invention. This would be undesirable in today's miniaturized sensors.
The present invention solves this problem by providing a circuit with two outputs, one of which is a primary output, representing the sensor signal and conducting when the sensor signal is present. The other output (secondary output) can be selected by the user to represent either the complement of the primary output or the status of the Alarm Signal, depending on the polarity of the DC supply voltage to the device. A diode bridge assures that the control circuitry receives voltage of the correct polarity regardless of the polarity of the supply voltage.